1. Field of the Invention
The present invention relates to an exposure apparatus for exposing a substrate to radiant energy and a method of manufacturing a device by using the exposure apparatus.
2. Description of the Related Art
Japanese Patent Publication No. 61-34252 describes an exposure apparatus which causes a photoelectric detector to detect light from an object to be exposed or the intensity of light that irradiates the object, obtains the output pulse of a frequency corresponding to the detection value, counts the number of pulses, and closes the shutter when the number of pulses has reached a predetermined value. The shutter has light-shielding portions and light-transmitting portions which are alternately provided on a rotating disk. Change between the shielded state and transmission state of illumination light is controlled by driving the shutter. It is necessary to correct an error in dose for irradiation of the object due to the delay time of shutter movement, that is, the time from generation of a shutter close signal to completion of shutter close. For this purpose, the number of pulses which corresponds to the dose during a delay time of the shutter movement is counted when the shutter is driven to the open state. The shutter close signal generation timing is corrected in consideration of the number of pulses.
In exposure control at a low dose, the arrangement described in Japanese Patent Publication No. 61-34252 may delay the shutter close timing. To prevent this, a method of closing the shutter without counting the pulses or a method of counting the pulses, as described above, in a lower light intensity state, and closing the shutter when the number of pulses has reached a predetermined value is employed. To reduce the light intensity, the light source position is moved in the optical-axis direction, or a neutral density filter is inserted between the light source and the object.
However, the method of closing the shutter without counting the pulses requires a technique of guaranteeing exposure of a substrate (wafer) or shot region at an appropriate dose. The present inventors found, through experiments, that the appropriate dose changes when the intensity of light reflected from a substrate held by a holder or the outer portion of the substrate changes. Hence, the dose can be too large or too small in the method of exposing the substrate to radiant energy for a predetermined exposure time.
The intensity of reflected light has a correlation with the area of the shot region on the substrate, the reflectance of the substrate, and the reflectance of the outer portion of the substrate. The area of the shot region on the substrate changes when the shot region falls outside the effective region of the substrate. When a reticle with a plurality of chip region patterns being arrayed is used, the shot regions are sometimes arrayed on the substrate such that some of the plurality of chip regions fall outside the effective region of the substrate. A shot region (to be referred to as a defective shot region) in which at least one chip region falls outside the effective region of the substrate necessarily has a smaller area than a shot region (to be referred to as a full shot region) in which all chip regions fall within the effective region of the substrate. This affects the intensity of light reflected from the substrate in exposure. The reflectance of the substrate can change in accordance with the type of photosensitizer or the layer under it. The reflectance of the outer portion (typically substrate chuck) of the substrate held by the holder depends on the material or coating of the outer portion.
An exposure control method under a lower light intensity enables accurate dose control without the above-described problem of excess or deficient dose, though the throughput is low.